This invention relates to a linear accelerator for use in linearly accelerating a beam of charged particles along a beam axis.
Heretofore, a wide variety of linear accelerators have been proposed so as to linearly accelerate a beam of charged particles, such as ions and electrons along a beam axis and may include, for example, an Alvarez type, a Wideroe type. They are simply and collectively called linac's. A radio frequency quadrupole linac is also included in such linear accelerators and is often abbreviated to an RFQ linear accelerator or an RFQ Linac.
In a conventional RFQ linear accelerator of the type described, the beam of charged particles is linearly accelerated and converged therein along a beam axis by the use of a quadrupole electric field. To this end, the RFQ linear accelerator comprises a conductive cylinder which surrounds a hollow space and has a cylinder axis coincident with the beam axis and a pair of ends having apertures positioned on the beam axis. In addition, first through fourth conductive vanes are arranged in the hollow space around the beam axis with an azimuthal angle of 90.degree. left between adjacent ones of the first through the fourth conductive vanes. These conductive vanes are extended along the beam axis and electrically connected to the conductive cylinder. A combination of the conductive cylinder and the conductive vanes may be referred to as a cavity resonator and is excited by an excitation device of a high frequency radio wave.
With this structure, it is possible to generate the quadrupole electric field around the beam axis and to linearly accelerate and converge the beam along the beam axis, as mentioned above.
More specifically, such a cavity resonator has a resonance mode of TE.sub.210 and is put into a resonant state when electric power which has a radio frequency equal to a resonance frequency of the TE.sub.210 mode is given from the excitation device. In this event, the quadrupole electric field is generated around the beam axis in the space gap among the vanes and is operable to linearly accelerate the beam.
However, the quadrupole electric field should be generated with a good symmetry around the beam axis and a good uniformity along the beam axis, and, otherwise, linear acceleration can not be accomplished. This means that inner edges of the conductive vanes must be accurately located at proper positions in the conductive cylinder. In other words, the conductive vanes should be strictly attached to the conductive cylinder and their locations should be finely adjusted.
In order to accomplish a uniform and good symmetrical quadrupole electric field, it is often pointed out that electric field tuning is effective. Such electric field tuning is made by the use of a plurality of electric field tuning devices which are projected into spaces partitioned by the conductive vanes and which are composed of metallic pieces. Specifically, a projected portion of each electric field tuning device is often varied in height to adjust the quadrupole electric field. Such electric field tuning devices make the linear accelerator intricate in structure. In addition, adjustment becomes troublesome for the linear accelerator.
Furthermore, various kinds of modes, such as TE.sub.11N modes, may also be induced within the space gap except the resonance mode of TE.sub.210 and have resonance frequencies somewhat different from and close to that of the resonance mode TE.sub.210. The quadrupole electric field is seriously disturbed as the resonance frequency of the resonance mode TE.sub.210 approaches the resonance frequencies of TE.sub.11N modes. In this connection, the resonance frequency of the resonance mode TE.sub.210 should be remote from the other resonance frequencies of TE.sub.11N. However, each resonance frequency of the above-mentioned modes is uniquely determined by a diameter and a length of a cavity. Therefore, inconvenience is caused to occur in that the length of the cavity can not be freely selected in the conventional linear accelerator.